Balantidium coli is thought to be the only pathogenic ciliate present in humans, as well as the largest protozoan parasite found in humans. This organism belongs to the class of ciliated protozoa, which can be found in a variety of hosts and is generally responsible for infecting humans.
Morphology of Balantidium coli
This organism, known as B. coli, is very large, with short cilia covering the entire circumference of the cell.
- The trophozoite stage can range in size from 30 to 120 m by 30 to 80 m, with larger versions reaching 90 to 120 m by 60 to 80 m.
- The trophozoite is tapered at the anterior end and has a cytostome attached.
- There are two nuclei visible: a large kidney bean–shaped macronucleus and a small nucleus known as the micronucleus.
- The cyst shape is elliptical and ranges in size from 45 to 65 m. Food vacuoles can be seen in the cytoplasm, and waste is eliminated through a small opening at the back of the organism.
Symptoms of Balantidiasis
Infection with B. coli can cause little to no symptoms in some people. Balantidiasis caused by B. coli infection causes a variety of symptoms and signs, including frequent and recurring diarrhea, stomach pain and cramping, vomiting, fatigue, and weight loss.
Life Cycle of Balantidium coli
Balantidium coli is primarily contracted via contaminated water and food, especially where there is animal contact. As in most protozoans, the cyst is the infective stage of Balantidium coli, and the host is infected through ingestion of the infective cysts by water and food. Protozoans in cyst form have a thick defensive membranous wall that renders them resistant to unfavorable environmental conditions. Excystation (hatching from a cyst) takes place in the small intestine, and reproduction is accomplished by binary fission. Cysts can grow in the colon’s lumen and infect other hosts. The trophozoite cannot infect humans or other animals, but it can replicate in the host through division and can cause tissue damage in the infected organism.
Infections are most common in places where some animals harboring the organism come into contact with humans. The parasite B. coli is commonly present in a wide range of mammals, including monkeys and pigs. In areas where humans treat pigs on a daily basis, infection rates have been estimated to be as high as 25% or higher of the general population. This organism is believed to be mainly transmitted from pigs, which tend to be the natural host for B. coli, as well as possibly a few other animal species, and humans become infected by interaction with these animals. However, there has been some evidence of human-to-human transmission of the infection, which may explain the 25% infection rate in areas where close contact with pigs is normal. The organism mainly affects the large intestine, where it is known to cause mucosal membrane lesions.
Laboratory Diagnosis of Balantidium coli
Microscopically, a wet mount of the organism B. coli exposes cilia that can be seen where they impact the parasite’s rotary motion. The large size and surrounding cilia in the trophozoite stage allow identification on a permanent slide of a stained fecal specimen or material obtained by sigmoidoscopic examination (endoscope inserted into the bowel). The two wall layers are ciliated but difficult to see in the cyst stage.
Treatment and Prevention
Consistency in personal hygiene and management of sanitary conditions is effective in avoiding B. coli infection. Although it has been theorized that there could be a cyst stage that is involved in fecal-oral transmission, care should be taken while handling monkeys or working with them in zoos and research laboratories where these animals may be used. However, pigs, as well as their raising and processing, are the source of the largest number of infections. Antibiotics such as metronidazole, iodoquinol, and tetracycline should be successful in clearing up the infection.